The present invention relates to a process and apparatus for cryogenically separating air to produce high purity argon. More particularly, the present invention relates to such a process and apparatus employing a three column distillation system in which argon is produced in an argon column having a sufficient number of theoretical stages to produce the high purity argon as a product.
Conventionally, argon is separated from air in a three column distillation system which consists of a high pressure column, a low pressure column and an argon column. In such a system, the high pressure column produces an oxygen rich liquid, the low pressure column further refines the oxygen rich liquid to produce an argon enriched mixture as a vapor, and the argon column refines the argon enriched mixture to produce crude argon as a tower overhead. In order to provide reflux for the argon column, a stream of the crude argon is condensed in a head condenser by a subcooled and expanded stream of the oxygen rich liquid from the high pressure column.
The crude argon contains oxygen and nitrogen which must be removed to produce high purity argon. Therefore, the crude argon is upgraded, generally through catalytic combustion to remove the oxygen followed by adsorbers to remove formed water and further distillation to remove nitrogen.
Theoretically, it is possible to increase the number of stages of separation within the argon column to enhance the separation of argon and oxygen. However, at least with argon columns employing trays or plates, this is not practical because the resultant pressure drop would lower the condensation temperature of the crude argon and therefore raise the degree of expansion required of the oxygen enriched liquid such that the oxygen enriched liquid would be at too low a pressure to flow into the low pressure column. The operating pressure range of the low pressure column cannot not be reduced to accommodate such a highly expanded oxygen enriched liquid because the crude argon feed flows from the low pressure column to the argon column under impetus of the pressure of the low pressure column.
There are prior art three column plants that are designed with a sufficient number of theoretical stages in the argon column to separate oxygen from the argon to an extent that catalytic combustion is not required in the upgrading of the crude argon. An example of this can be found in U.S. Pat. No. 5,019,145 in which 150 theoretical stages are employed in an argon rectification column utilizing low pressure drop packings. The use of such packings prevents the excessive pressure drop that would otherwise occur with plates or trays.
U.S. Pat. No. 5,133,790 is an example of cryogenic rectification process and apparatus in which both oxygen and nitrogen concentrations are directly reduced so that a high purity argon product can be withdrawn directly from the argon column without subsequent catalytic and distillation stages. In this patent, the low pressure column is operated with a sufficient number of theoretical stages (provided by structured packing) such that the nitrogen concentration in the feed to the argon column is less than 50 parts per million. Since less nitrogen is being fed to the argon column, there will be a lower concentration of nitrogen in the argon produced in the argon column. In order to remove the oxygen, the argon column can be fabricated with structured packing to provide approximately 150 theoretical stages, as called for in U.S. Pat. No. 5,019,145, to effect the degree of oxygen separation required for the production of the high purity argon product.
The prior art patents, discussed above, both depend on the use of a low pressure drop packing in at least the argon column to prevent excessive pressure drop. As will be discussed, the present invention provides a process and apparatus for producing a high purity argon product directly from the argon column that does not depend on structured packing for its operability. In fact, both the argon and low pressure columns can be conventionally designed with sieve trays, a low pressure drop packing or any other type of liquid-gas contact device or any combination thereof. Further advantages of the present invention will become apparent from the following discussion.